6.9L International IDI Diesel

6.9L IDI Diesel Specs, History, & Information

International and Ford entered an agreement in 1981 so that Ford could offer the 6.9L IDI diesel in their F-250 & F-350 pickup truck models. Development for the 6.9 can be traced back as far as 1978 by International Harvester, who would later sell their "Harvester" agricultural business to Case to focus on the medium/heavy duty truck and engine market; the company was renamed (and is still known today as) International Navistar. The 6.9L IDI was offered by Ford for the 1983 model year with 170 horsepower rating. It was such an instant success that Ford had to ask International to increase production. Naturally aspirated, the engine wasn't particularly known for speed. But it was everything it was designed to be, and possibly even more; a solid, reliable workhorse that could move an impressive amount of weight from point A to point B. It was also easy on fuel, a positive selling point considering the gas guzzling V-8's that it was competing against on the dealership lot.

In the long run, the 6.9L would prove to be dependable and superior in longevity to its primary competition of the era. The 6.9L IDI was backed by either a T-19 4 speed manual or C6 3 speed automatic transmission, with the exception of some rare ZF S5-42 5 speed manual transmissions in the 1987 model year. It was eventually replaced by the 7.3L IDI, which is essentially a redesigned, bored over, higher output version of the engine.

1984 model year and newer engines received a higher compression ratio than the introductory engine. Pre-84 engines are referred to as "A" model engines, while the later was designated the "B" model. All engines with serial number 59209 and greater were model B engines. In addition to the increased compression ratio, B model engines had an alternate exhaust valve and prechamber insert design (smaller prechamber). The piston height was increased by 0.003" to achieve the higher compression ratio, and the piston design also had an enlarged cup to promote a more efficiency swirl.

The 6.9L IDI was ultimately successful in competing with GM's 6.2L Detroit diesel, which was designed specifically as an economical alternative to their line of gas engines and not a heavy hauler. The 6.9 proved more reliable and more conducive to the true purpose of owning a diesel than their competition. To respectfully judge the 6.9L IDI today, it is impossible to compare to modern diesel engines. If nothing else, the 6.9 was technologically limited, as it was completely mechanically controlled, naturally aspirated, and was developed at a time when diesels were not in the horsepower/torque race that they have been in more recent times. It is quite common to turbocharge the 6.9 via aftermarket kits. While this greatly enhances performance an can even improve fuel economy, the only safe way to do so without challenging the engine's reliability or longevity is to run conservatively low boost pressures and properly calibrate the injection pump.